The present invention relates to a prewarning device for breakouts of molten metal on ceramic furnace linings of melting furnaces, particularly induction melting furnaces with electrodes or electrode networks arranged on the corresponding furnace lining, such as, in particular, on its outer side. The electrodes are divided into two groups of different polarity, spaced apart and adapted to be connected to an evaluation unit in order to determine the electrical, temperature-dependent resistance of the furnace lining between the two groups of electrodes.
In melting furnaces of this type, the ceramic furnace lining is subject to very strong thermal, chemical and mechanical stresses in operation. In this way, wash-outs and possibly cracks are formed which can extend in an induction melting furnace up to the inductor. If penetration of the molten metal into the inductor is not recognized at a sufficiently early time, the furnace can experience considerable damage and, in extreme cases, there may even be an explosive emptying of the melting unit.
In order to be able to note such defects in induction melting furnaces at an early time, prewarning means of the above type are known which utilize the principle of resistance measurement. This is based on the realization that the electrical resistance between any two contact points of the ceramic furnace lining, for instance on the outer side, is dependent on the temperature, in the manner that it decreases considerably with an increase of temperature, namely by a few powers of ten in the high-ohmic region. If a breakout point is established in the wall of the furnace lining, a local increase in temperature takes place which, with a suitable arrangement of the electrodes on the furnace lining and on its outer wall, can be noted via the resistance measurement. The problem is in being able to arrange the network of electrodes sufficiently close together on the outside of the ceramic furnace lining so as to be able to obtain a reliable early warning of the threatened breakout of the melt.
In one known prewarning device of this type for an induction melting furnace, the network of electrodes is arranged in grooves on the outside of a finish crucible which is introduced in an induction furnace as a prefabricated part and surrounded by a ceramic back-tampering material in order to fill the necessary annular slot towards the inductor wall. In this connection, the network of electrodes lies at a sufficient distance in front of the inductor wall to be able to signal in due time a threatened point of breakout without damage to the inductor. In actual practice, however, the use of finish crucibles is very limited, because a conventional furnace lining is generally preferred in which the furnace lining is built up from the ceramic material at the place of use.
With this known prewarning device, considerable difficulties exist in installing the network of electrodes for the signalling of breakout points at reasonable expense in such a manner that a reliable and accurate early recognition of a potential breakout place is possible. In the case of a conventional lining, recourse has been had to the use of bar electrodes which are arranged in vertical alignment between a heat-insulating layer and the crucible material along the circumference of the crucible, as to which see Federal Republic of Germany Patent 27 18 016-A1, in which the reduction in the electrical resistance of the at least partially sintered ceramic crucible material between the adjacent electrodes as a result of temperature increase is evaluated. The disadvantage of this known device is that there is danger of a short circuit of the inductor voltage, as well s an influencing of the measurement voltage over the vertical electrodes in the case of a wet heat-insulating layer. Also, the installation and the connection of the electrodes are difficult. This known device also has shortcomings with respect to the reliability of the indications, because the advancing molten metal is recognized relatively late.
Accordingly, it is a main important object of the present invention to provide a prewarning device of the aforementioned type which permits simple application of the electrodes on the outside of the furnace lining and assures a high dependability of indication on the part of the entire system.